Etched surface gratings fabricated using computed interference between simulated optical signals and reduction lithography

Abstract

An optical apparatus comprises a set of diffractive elements on a substrate. They are arranged: (i) to receive an input signal propagating from an input port as a diffraction-guided optical beam, (ii) to diffract a portion of the received input signal as an output signal, (iii) to route the output signal to propagate to an output port as a diffraction-guided optical beam, and (iv) to exhibit a positional variation in diffractive amplitude, optical separation, or spatial phase over some portion of the set. The arrangement of the diffractive elements corresponds to an interference pattern derived from computed interference at a surface of the substrate between a simulated design input and output optical signals. Each diffractive element comprises at least one trench segment positioned along a path defined by a constant-phase contour of the interference pattern. Each trench segment is substantially rectangular or trapezoidal in transverse cross section.

1. An optical apparatus, comprising a set of diffractive elements on a substrate that are arranged: (i) to receive at least a portion of an input optical signal propagating from an input optical port as a diffraction-guided optical beam, (ii) to diffract a portion of the received input optical signal as an output optical signal, (iii) to route the output optical signal to propagate to an output optical port as a diffraction-guided optical beam, and (iv) to exhibit a positional variation in diffractive amplitude, optical separation, or spatial phase over some portion of the set, wherein the arrangement of the diffractive elements corresponds to an interference pattern derived from computed interference at a surface of the substrate between a simulated design input optical signal and a simulated design output optical signal, 
       wherein:
 each diffractive element comprises at least one trench segment positioned along a path defined by a corresponding constant-phase contour of the interference pattern; and 
 each trench segment is substantially rectangular or trapezoidal in transverse cross section. 
 
     
     
       2. The apparatus of  claim 1 , wherein the trench segments are formed on the substrate surface by binary projection lithography. 
     
     
       3. The apparatus of  claim 2 , wherein the arrangement of the diffractive elements includes (i) relative widths of each trench segment or an adjacent rib, (ii) depth of the trench segments, and (iii) displacement of each trench segment relative to the corresponding constant-phase contour, the depth, relative widths, and displacements at least in part determining relative diffractive strength of each diffractive element. 
     
     
       4. The apparatus of  claim 3 , wherein each trench segment is substantially rectangular in transverse cross section, so that the diffractive element set formed on the substrate exhibits relatively enhanced diffraction efficiency near a Littrow arrangement of the input and output optical ports relative to the substrate. 
     
     
       5. The apparatus of  claim 1 , wherein the substrate surface is substantially planar. 
     
     
       6. The apparatus of  claim 5 , wherein the diffractive element set is arranged so that the respective wavefronts of the input and output optical signals exhibit differing convergence, divergence, or collimation properties. 
     
     
       7. The apparatus of  claim 1 , wherein the substrate surface or the diffractive element set is reflective over a desired operational wavelength range, the diffractive elements and the substrate thereby comprising a reflective surface grating. 
     
     
       8. The apparatus of  claim 7 , further comprising a coating formed over the diffractive element set and the substrate surface so as to impart reflectivity over the desired operational wavelength range. 
     
     
       9. An optical spectrometer, comprising:
 an input optical port for receiving an input optical signal into the spectrometer; 
 an output optical port for transmitting an output optical signal out of the spectrometer; and 
 an optical surface grating comprising a set of diffractive elements on a substrate that are arranged: (i) to receive at least a portion of an input optical signal propagating from an input optical port as a diffraction-guided optical beam, (ii) to diffract a portion of the received input optical signal as an output optical signal, (iii) to route the output optical signal to propagate to an output optical port as a diffraction-guided optical beam, and (iv) to exhibit a positional variation in diffractive amplitude, optical separation, or spatial phase over some portion of the set, wherein the arrangement of the diffractive elements corresponds to an interference pattern derived from computed interference at a surface of the substrate between a simulated design input optical signal and a simulated design output optical signal, 
 
       wherein:
 each diffractive element comprises at least one trench segment positioned along a path defined by a corresponding constant-phase contour of the interference pattern; and 
 each trench segment is substantially rectangular or trapezoidal in transverse cross section. 
 
     
     
       10. A method, comprising:
 formulating a simulated design input optical signal propagating toward and through a diffracting volume from a designed optical input port as a diffraction-guided optical beam; 
 formulating a simulated design output optical signal propagating through and away from a diffracting volume to a designed optical output port as a diffraction-guided optical beam; 
 computing an interference pattern between the simulated input and output optical signals over a design diffracting surface in the diffracting volume; 
 computationally deriving an arrangement of a set of diffractive elements on the design surface from the computed interference pattern, so that when the diffractive element set is formed on a substrate surface substantially conforming to the design surface, the diffractive element set would route, as an output optical signal propagating to the output optical port, a diffracted portion of an input optical signal propagating from the input optical and incident on the diffractive element set that is diffracted by the diffractive element set, wherein the arrangement of diffractive elements exhibits a positional variation in diffractive amplitude, optical separation, or spatial phase over some portion of the set; 
 forming at a desired mask size scale a mask with a mask pattern corresponding to the derived arrangement of the diffractive element set; 
 forming the diffractive element set on the substrate surface by projecting the mask pattern onto the substrate surface at a desired projection size scale and then forming the diffractive elements as a set of corresponding trench segments having substantially rectangular or trapezoidal transverse cross sections. 
 
     
     
       11. The method of  claim 10 , wherein the product of the mask size scale and the projection size scale is substantially equal to unity. 
     
     
       12. The method of  claim 10 , wherein the product of the mask size scale and the projection size scale differs substantially from unity. 
     
     
       13. The method of  claim 10 , wherein the diffractive elements are formed on the substrate surface by binary projection lithography. 
     
     
       14. The method of  claim 13 , further comprising forming multiple replica diffractive element sets on multiple corresponding substrate surfaces by stamping, embossing, pressing, or molding employing the first diffractive element set as a master or employing a secondary master formed from the first diffractive element set. 
     
     
       15. The method of  claim 13 , wherein the arrangement of the diffractive elements includes (i) relative widths of each trench segment or an adjacent rib, (ii) depth of the trench segments, and (iii) displacement of each trench segment relative to the corresponding constant-phase contour, the depth, relative widths, and displacements at least in part determining relative diffractive strength of each diffractive element. 
     
     
       16. The method of  claim 15 , wherein each trench segment is substantially rectangular in transverse cross section, so that the diffractive element set formed on the substrate exhibits relatively enhanced diffraction efficiency near a Littrow arrangement of the input and output optical ports relative to the substrate. 
     
     
       17. The method of  claim 10 , wherein the substrate surface is substantially planar. 
     
     
       18. The method of  claim 17  wherein the diffractive element set is arranged so that the respective wavefronts of the input and output optical signals exhibit differing convergence, divergence, or collimation properties. 
     
     
       19. The method of  claim 10 , wherein the substrate surface or the diffractive element set is reflective over a desired operational wavelength range, the diffractive elements and the substrate thereby comprising a reflective surface grating. 
     
     
       20. The method of  claim 19 , further comprising forming a coating over the diffractive element set or the substrate surface so as to impart reflectivity over the desired operational wavelength range.